This invention relates to specially constructed fuel hose for use in supply of automotive fuels, and particularly to such hose as used in automotive vehicles.
Regulation has been adopted in California, and are under active consideration elsewhere, which will require in a short time that new automotive vehicles sold in the state must pass a vehicle permeation test presently designated as the S.H.E.D. TEST, which test is designed to measure the emissions (i.e. vapors) from vehicles with the engine not running. Within a few years 100% of new vehicles sold in that state will be required to pass this standard. A maximum of 2 grams of vapor emission per 24 hours period is allowable. Obviously, these emissions will come from vaporization of fuel permeating the fuel hoses and other parts of the vehicle fuel supply system.
In addition to more stringent requirements for low emissions from static vehicles, automotive exhaust emission standards are becoming more stringent, and one of the efforts to meet such requirements is through the use of various blends of so-called "clean burning" fuels, which include large percentages of methanol or ethanol.
It is believed that these regulations may become Federally mandated, and therefore automotive manufacturers are seeking from parts suppliers various components with lower emission values and (as related to fuel hose) improved permeation resistance. Traditional solutions to improved fuel hose permeation resistance involve selecting specific rubber compounds which are known to exhibit low permeation characteristics and compounding those with other ingredients to achieve desired results. With the variety and mixture of fuels likely to be used in the near future, more stringent requirements are being placed on the performance of fuel hose. The rubber compounds can be tailored to give certain minimum permeation for a particular fuel, but unfortunately these levels are still well above anticipated future standards. In addition, a compound tailored for minimum permeation when subjected to one specific fuel will often exhibit marginal results when subjected to other fuels or combinations of them.
More recent solutions to fuel hose permeation problems involve the use of fluoroelastomers, which generally exhibit very low permeation values even for the variety of fuels mentioned above. However, there are drawbacks inherent in the use of fluoroelastomers in an efficient cost effective hose production environment. These polymers are very expensive, ten to twenty times as expensive as traditional rubber polymers used for hoses. For this reason fluoroelastomers are normally produced by extrusion as a very thin wall tube in a co-extrusion process which also simultaneously forms a thicker layer of low permeability rubber compound around the fluoroelastomer tube. It is important that these two co-extruded tubes have good adhesion to each other so a resulting product made from them can be processed through the remaining manufacturing steps and insure good integrity of the finished product, to withstand normal in-service abuse. Unfortunately, fluoroelastomers do not have tack (surface stickiness) in their uncured state to permit them to bond to adjacent layers of uncured rubber. Processing difficulties occur when these layers do not adhere together, resulting in unacceptable amounts of scrap material.
Additional processing difficulties arise as a direct result of the cost of fluoroelastomers and the interest in minimizing the amount used. Thicknesses of 0.020 to 0.040 inch are sufficient to achieve low permeation resistance levels, but because of the small opening in the extrusion tool (die) needed to produce such thickness, difficulties arise from inevitable foreign material found in rubber compounds, which cause die openings to plug. This can result in a tear in the fluoroelastomer layer, allowing a direct path for passing fuel to the outer layer of higher permeation rubber. Because the inner co-extruded fluoroelastomeric layer cannot be seen during the production process, considerable scrap can be encountered or defective product is made and not easily discovered.
Fluoroelastomer compounds tend not to be viscous at the typical processing temperatures for rubber compounds. This property, combined with the need for small opening in the co-extrusion die to conserve the expensive material, results in high back pressure in the extruder. The extrusion equipment is thus placed under greater stress, which contributes to more rapid wear of the equipment.